Permanent magnet synchronous motors (PMSM) are widely used in motion controls, electric vehicles, and industrial turbo generators (ITG). PMSMs are typically coupled with engines to provide assistance to the engines at lower engine speeds. Because a turbo engine cannot start itself at zero speed, PMSMs using a starter inverter are needed to assist the turbo engine in accelerating to about half of its rated speed. At this speed, the turbo engine has sufficient torque to be driven to its rated speed without a PMSM.
To start a turbo engine, and specifically an industrial turbo engine, a starter inverter drives the PMSM, which is coupled to the turbo engine, to an initial desired speed to warm up the turbo engine. The starter inverter, which converts direct current (DC) into alternating current (AC), takes power from either batteries or a rectified grid. After the turbo engine is warmed, the drive is then increased to a second desired speed, typically about half of the rated speed of the turbo engine. The turbo engine will then take over the process without the aid of the AC motor and continue accelerating to nominal speed.
An "open-loop" volts per hertz control method is a commonly used AC motor control method. The volts per hertz (V/f) algorithm is used to run the motor at variable speeds. The V/f algorithm is very simple and inexpensive to implement in comparison to more sophisticated AC motor control algorithms. The "open-loop" control method varies the three-phase output frequency by adjusting the slope, or the ratio between frequency and voltage. Normally, a single straight-line V/f ratio is determined that drives the AC motor to the turbo uncoupling speed, where the turbo engine can produce sufficient torque to accelerate on its own to a desired speed without the aid of an AC motor. No sensing equipment is needed, the calculations are simpler and a less powerful, less expensive motor control unit (MCU) can be used.
One problem with the "open-loop" control method is that due to a possible wide range of DC bus voltages and the short accelerating time, typically about 12 seconds, for an industrial turbo engine to accelerate from 0-33,000 rpm, a single straight-line voltage per hertz ratio calculated by the "open-loop" method does not give a satisfactory dynamic response. In addition, the "open loop" control method often results in larger line current outputs at high speeds, or unacceptable steady state current at low speeds if a short accelerating time is required.
It is thus an object of the present invention to determine a control method for the high-speed PMSM that overcomes the problems associated with the open-loop control methods.